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Orion Pax, Part 1

Optimus Prime loses all memory of being a Prime after using the Matrix of Leadership to send Unicron back into stasis. Megatron takes advantage of this and takes "Orion"(Optimus's name before becoming a Prime) on to the Nemesis, convincing him the Decepticons are good and the Autobots are evil (Before becoming a Prime, Optimus and Megatron were close friends).